What is a good way to learn clean topology?
A good way to learn to use clean topology is to do a lot of modelling
'Clean' topology is topology that is easy and convenient to work with. We avoid triangles because they do not allow the use of a lot of functions related to edge loops or do not look nice after surface subdivision, we avoid non-manifold geometry because some algorithms are confused by it and do not work as expected, we use as little of geometry as possible to define shapes and forms because it is easier to work with and there is less lag, we use denser geometry where objects are going to deform because they do not look nice when deformed otherwise and so on and so on. Clean topology has natural reasons to be - that's why you learn it naturally while modelling. If you model something for animations, you need to take extra care to understand the rigging and animation process enough to have suitable topology, but else than that, most of the time the more modelling you do, the better you get at judging what topology is 'clean'.